The objectives of the proposed research are to reveal and explain how the T4 genome is organized, why it is organized that way, and how the sequential expression of the genome is regulated. This is being approached by isolating and studying T4 mutants affecting genes that determine related enzymes, namely those involved in the biosynthesis of thymidylic acid in T4-infected Escherichia coli. Some of these mutants are being recognized by their resistance or sensitivity to inhibitors of nucleotide metabolism and are providing new insights into both T4 regulation and mechanisms of drug action and resistance. It has been shown that the T4 genes cd, nrd, td, and frd, which control the phage-induced production of deoxycytidylate deaminase, ribonucleotide reductase, thymidylate synthetase, and dihydrofolate reductase, are very closely linked and may be part of a genetic unit under common regulatory control. Attempts are being made to further characterize these genes and genes controlling other enzymes involved in thymidylate synthesis by isolating and studying T4 mutants in these genes. The study of mutants in these genes, which control related functions, will hopefully lead to a better understanding of the relation of the genes to each other and the regulation of their expression.